Ferrite-loaded notch antenna



Oct. 13, 1970 P. F. STANG 3,534,370

FERRITE-LOADED NOTCH ANTENNA FiledAug. 9, 1968 5 Sheets-Sheet 1 '4 |,1I ,L'H

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FIG. 2

NV PAUL F. STANG 2i BY 1" Age Oct. 13, 1970 P. F. STANG 3,534,370

FERRITE-LOADED NOTCH ANTENNA Filed Aug. 9, 1968 5 Sheets-SheetB INVENTOR. PAUL F. STANG Age i United States Patent O 3,534,370 FERRITE-LOADED NOTCH ANTENNA Paul F. Stang, Saugus, Calif., assignor to Lockheed Aircraft Corporation, Burbank, Calif.

Filed Aug. 9, 1968, Ser. No. 751,498 Int. Cl. H01q 1/00, N28

US. Cl. 343-108 14 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Prior attempts to employ ferrites in conjunction with magnetic antennas have been generally unsuccessful. Notch antennas are desirable for use with airborne radio transmitters since their flush-mounted configuration is aerodynamically optimum. However, conventional design parameters for the very high frequency (VHF) range define antennas which are unacceptably large for use in helicopters and tactical support aircraft. By means of the present invention there is provided a novel and improved antenna coupler which substantially reduces the size of the required notch structure and also provides other benefits, as will be described hereinafter.

As is well known, the conventional notch-excited antenna circulates radio frequency (RF) currents in the aircraft structure surrounding the notch. A ferrite element, located near the front of the notch in accordance with the present invention, minimizes the necessary notch size as compared with prior designs and also broadens the bandwidth of the antenna. Multiple ferrite exciters may be employed for still greater bandwidths, in accordance with the invention.

It is therefore an object of the invention to provide an efficient flush-mounted transmitting antenna for small airborne vehicles.

Another object of the invention is to provide a novel and improved antenna coupler for notch-type antennas.

Still another object of the invention is to provide a ferrite-loaded notch exciter for use in notch-excited aircraft antennas.

Yet another object of the invention is to provide a novel and improved antenna incorporating an antenna coupler which may be tuned by means of a direct current (DC) control voltage.

These and other objects of the invention will become better understood upon consideration of the accompanying specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a rotary wing compound aircraft in which two antennas, constructed in accordance with the invention, are installed.

FIG. 2 is a perspective view of the antenna notch and its ferrite coupler.

FIG. 3 is a schematic circuit diagram of a first embodiment of the invention employing switched-capacitor tuning.

FIG. 4 is a schematic wiring diagram of the remotelycontrolled selective switching system used with the tuning and matching capacitor arrangement employed in the embodiment of FIG. 3.

FIG. 5 is a schematic circuit diagram of an alternative embodiment of the invention employing variable bias tuning.

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DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 there is shown by way of example a pair of antennas, constructed in accordance with the invention, installed in a small aircraft 1. Actually it is the conducting surface of the aircraft 1 which serves as the radiating portion of the antennas by reason of the RF currents which are caused to circulate on the aircrafts surface. The invention relies upon a coupling de vice which is utilized to propagate RF currents from the region of a notch in the airframe, such as in the vertical stabilizer 2, in the case of exciter 3 and in the left Wing 4- in the case of exciter 5. The notch exciter induces a high current flow around the notch, which current spreads out over the entire surface and causes it to radiate the RF signals. A ferrite loading element comprises the inductive portion of the coupling device incorporated into the notch of each antenna and allows a substantial reduction in size over that of unloaded linear antennas heretofore employed for generally similar purposes. The coupling device will be referred to hereinafter as a fer rite notch exciter.

In the embodiment shown in FIG. 1, two ferrite notch exciters 3 and 5 are flush mounted in the trailing edge of the vertical stabilizer 2, and the wing 4, respectively. The notch-excited antennas 3 and 5 can be employed efliciently in the RF band from 30 megahertz to 76 mega hertz. The physical dimensions of the antenna vary according to the type of aircraft in which it is to be installed. The conducting surface of the aircraft 1 can serve as an efficient radiator providing its conductivity is sufiiciently high and the size of the airframe is an appreciable fraction of the wavelength of the RF signal to be radiated. Radiation due to RF currents in the region of the notch area is negligible since the major portion of the radiation emanates from those surfaces which are excited by the notch. For example, when the notch is placed in the tail section 2 most radiation emanates from the tail surface. At lower frequencies where the fuselage becomes resonant to the induced wavelength, the radiation will emanate mostly from the fuselage.

The location of the antenna will influence its polarization. Thus, if it is desired to operate with vertical polarization, then it would be appropriate to excite the vertical stabilizer 2 of the aircraft. The cutout or notch location may be located in either the leading or trailing edge of the vertical stabilizer 2. In the case of horizontal polarization it is useful to excite the Wing 4 of the aircraft. The cutout or notch should be located in the trailing edge of the wing 4 near the fuselage. This may be readily accomplished in a compound vehicle of the type shown in FIG. 1 since there are no movable control surfaces on the wing.

Referring to FIG. 2, the elements comprising the notch exciter are shown and include ferrite cylinders 6 and 7, tuning capacitors 8-11, matching capacitors 13-16, and the relay system 17.

The notch itself may comprise an integral metal frame 18 having extended flange portions (typical ones of which are indicated at 2124) for improving the rigidity of the structure and to facilitate mounting the device in the airframe. Standoff insulators 25-26 extend downwardly from the upper edge of the notch frame 18 and carry the conductive mounting bracket 27 which in turn supports the tuning and matching capacitors 811 and 13-16. The ferrite cylinders 6 and 7 extend between the lower edge of the bracket 27 and the interior surface of the lower end of the frame 18. Non-magnetic conductive slugs extend through the hollow center of cylinders 6 and 7 and complete a circuit between bracket 27 and the lower wall of the frame 18 (as viewed in FIG. 1). A

suitable transmission line connector 28 may be mounted in the upper end of the frame 18. It should be understood that a dielectric cover sheet may be employed which extends over the otherwise open sides of the notch to maintain the aerodynamic surface integrity of the aircraft 1. The described dielectric cover is shown installed in FIG. 1 but has been omitted from FIG. 2 for clarity; it may comprise fiberglass lamina or similar sheet-dielectric material.

The embodimentshown in FIG. 2 employs two ferrite cylinders 6 and 7 in order to obtain desired operating parameters. It should be understood, however, that a single cylinder, or more than two cylinders may be called for in any particular antenna coupler circuit. The selected ferrite element, or elements, may be tuned by selectively switching capacitors 8-11 and 13-16 into, or out of, the circuit. As is known to those versed in the art, ferrite is a magnetic material and it is therefore necessary to avoid any spurious magnetic coupling to the ferrite. Thus, the coupling parameters between the ferrite coupler and the transmission line 28 must be carefully controlled. Inductive coupling between the transmission line and the ferrite cylinders 6 and 7 should be avoided to prevent erratic operation. Either direct coupling or capacitive coupling may be employed. In the embodiment shown in FIG. 2 through 5, capacitive coupling is employed. It should be understood, however, that direct coupling may be employed where application parameters so indicate, in which case the coupling capacitors would be omitted.

The active inductive component of the antenna circuit is a function of the ferrite material used. Ferrite materials have the property of changing their effective permeability (,u.) with changes in frequency. The ferrite material is selected to obtain one with moderate losses and frequency dependence, thereby obtaining desired variations of the inductive component. The ferrite material acts as a limited self-tuning device. An essential feature of the invention is that the ferrite influence the inductive component of the notch circuit in consonance with the applied frequency. Moderate losses in the ferrite broaden the bandwith as would a load resistor in the circuit. Also, at higher frequencies the effective p. of the ferrite decreases, which changes the inductance of the notch and therefore broadens the frequency band.

In a practical implementation of the invention various types of ferrite material may be used including those identified as magnesium-iron TTIOS, nickel-iron TIZ- 130 and Ferroxcube type IVD. Magnesium ferrite and nickel ferrite are similar in frequency sensitivity and therefore allow operation over the same band. The Ferroxcube material has been found to have the greatest frequency dependence and allows greater bandwidth of operation but at higher magnetic losses and, consequently, lower efficiency.

Although there is a slight reduction in the power transfer efficiency resulting from the use of ferrites, which efiiciency reduction is attributable to magnetic losses, the advantages outweigh the losses. Specifically, the ferriteloaded notch is simpler in construction, has a greater bandwidth and is of considerably smaller size. Furthermore, it allows fixed tuning and matching over practical bandwidths without expensive or complex matching devices.

Due to the inherent wide bandwidth of the antenna, continuous tuning is not necessary; however, step tuning is desirable for large changes in operating frequency. As mentioned previously, step tuning is employed in the exemplary embodiment and is accomplished by means of a relay-switch capacitor tuning bank. The relays may be remotely operated in conjunction with the transceiver to which the antenna is coupled for selecting the proper frequency band of operation and matching the transmission line to the ferrite excited impedance.

The combination of the notch and the coupling device forms a closed circuit or loop which is analogous in operation to the well-known slot antenna. However, a notch-excited device constructed in accordance with the invention does not exhibit the same radiation characteristics as a loop or slot.

In a preferred construction of the present invention, the ferrite elements 5 and 6 are placed at the front section of the notch, as opposed to the rear or closed end of the notch, since this forward area contains the major portion of the circuits inductance. Also, if the ferrite elements were to be placed at the back of the notch, the surrounding metal of the airframe would practically eliminate the desired effect of the ferrite. Practical structural considerations will dictate how far forward in the notch, the ferrite elements may be placed.

Typically, a ohm transmission line is employed. The voltage standing wave ratios (VSWR) have been found not to exceed a value of 2.5 :1 in each of the following four selected frequency bands: 30 megahertz to 36 megahertz 36 megahertz to 46 megahertz 46 megahertz to 5 6 megahertz 56 megahertz to 76 megahertz In a practical construction of the invention, designed to operate in the FM/VHF communication frequency range from 30 mHz. to 76 mHz., the length of the notch frame may be approximately 8.0 inches high and 6.5 inches deep. The width may be contoured to the shape of the airframe structure, such as the vertical tail fin for flush mounting. The number of ferrite cylinders employed is determined by the required inductance of the circuit. In the aforementioned construction, three nickel-iron ferrite cylinders are used, each having a length of 6.0 inches with two having diameters of 1.0 inch and the third having a diameter of 1.75 inch.

Referring to FIG. 3 there is shown a schematic wiring diagram of a remotely tuned antenna employing a capacitive tuning bank for fine tuning. Capacitors 31-34 comprise tuning capacitors and capacitors 35-38 comprise matching capacitors. Contacts 39-44 are controlled by associated relay coils (described hereinafter in connection with FIG. 4) for switching selected ones of the tuning capacitors and matching capacitors into the active circuit. The central conductor 45 of the transmission line 46 connects to contacts 39-41. The outer conductor 47 of the transmission line 46 connects directly to the notch frame 48. Contacts 42-44 connect selected ones of matching capacitors 35-38 between the notch frame 48 and plate conductor 49. Ferrite cyinders 51 and 52 are interposed between notch frame 48 and plate conductor 49, near the forward end of the notch. Each of the ferrite cylinders 51 and 52 is provided with a centrally disposed aluminum slug 53 and 53 respectively, which completes the circuit to the upper end of the notch and comprises the series inductive component of the antenna loading circuit. Dielectric 'window 55 encloses the forward end of the notch.

Referring to FIG. 4 there is shown a schematic wiring diagram of a remotely-controlled relay system suitable for selectively adjusting the operating frequency of the antenna of FIG. 3, in discrete steps. The RF input to the antenna is supplied via conductor 45 to capacitor 34 and relay contacts 39-41. Capacitors 35-37 may be selectively connected to ground (viz., the portion of the notch frame 48terminating at conductor 47 of the transmission line) via respective ones of contacts 42-44. Relay coils 56-58 have a common power supply lead 59.

Power supply 61 provides an operating potential to energize selected ones of relay coils 56-58. One terminal of power supply 61 connects to lead 59 and the remaining terminal connects to the arm contact 62 of band selector switch 63. The switch 63 may be conveniently located in the vicinity of the radio transmitter to which the antenna is connected, and permits any given one of relay coils 56-58 to be energized.

As can be seen, when all three relay coils (5658) are de-energized, the antenna will be tuned to a first operating frequency by means of tuning capacitor 38 and impedance matching capacitor 34. A second operating frequency may be obtained by energizing the first relay coil 56 to add capacitors 31 and 35 to the circuit.

Energizing relay coil 56 through switch contact 64 will cause contacts 39 and 42 to close thereby connecting capacitor 31 to the transmission line 46 and shunting capacitor 35 to ground 48, thereby suitably changing the matching impedance of the antenna and the antenna tun- Energizing the second relay coil 57 via switch contact 65 will add tuning capacitor 36 and matching capacitor 32 to the circuit. Similarly, energizing relay coil 58 via switch contact 66 will add tuning capacitor 37 and matching capacitor 33 to the circuit. Additional band steps may be obtained by simultaneously operating two or more relays to further increase the matching and tuning capacitances, as will be apparent to those versed in the art. The common leads of capacitors 31-38 connect to the plate conductor 49 as shown in FIG. 3.

As will be apparent to those versed in the art, the relays 56-58 may be physically located at the site of the antenna notch.

There is shown in FIG. 5 an alternate method of tuning the antenna. All but one turning capacitor and one matching capacitor employed in the embodiment of FIGS. 3 and 4 are omitted and a D-C bias tuning arrangement is employed. The transmission line 46 passes through an aperture in the notch frame 48 and has its outer conductor 47 connected to the frame 48. The central conductor 45' connects to one terminal of matching capacitor 67; the remaining terminal of the capacitor connects to the junction between tuning capacitor 68 and plate 69. Capacitor 67 has one of its terminals returned to the grounded portion of the notch frame 48. Ferrite cylinder 71 is interposed between plate 69 and notch frame 48'. Metal slug 72 completes a circuit from plate 69 to the upper side 73 of the notch frame 48. As can b seen in FIG. 5, a tuning coil 74 is coaxially disposed about the lower end of cylinder 71. Leads 75 and 76 connect the coil 74 to a D-C power supply 77 via rheostat 78. Control rheostat 78 selectively varies the current to the coil 74 thereby selectively modifying the magnetic flux produced by the coil 74. The magnetic flux produced by coil 74 modifies the inductance of the ferrite cylinder 71 and thereby changes the value of the inductive component of the antennas loading circuit. This in turn alters the resonant frequency of the antenna. In a typical construction the output of the D-C power supply 77 may be varied from 0 to 28 volts via the control 78.

From the foregoing it will be seen that the present invention provides an antenna meeting the objectives set forth hereinabove and which is readily adapted to installation in small aircraft to replace helix and dipole antennas, used heretofore, thereby overcoming the limited effectiveness and excessive drag of such prior devices.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated may be made by those versed in the art. For example, the orientation of the notch structure with respect to the airframe may be altered in accordance with specified aerodynamic parameters and radiation patterns. Furthermore, the notch dimensions may be increased or decreased in accordance with particular mechanical and electrical design requirements. Such modifications may be made without departing from the spirit of the invention; therefore, it is intended that the invention be limited only as indicated by the scope of the following claims.

What is claimed is:

1. In a vehicle having an electrically conductive exterior surface, an antenna comprising:

means defining a notch in said surface;

a source of radio-frequency currents;

inductance-capacitance coupler means for coupling radio-frequency currents from said source to said notch-defining means and to said conductive exterior surface, the inductive portion of said inductancecapacitance means including a ferrite material, the inductance of which changes in response to changes in the frequency of said radio-frequency currents, thereby resulting in consonant changes in the tuning of said antenna.

2. An antenna as defined in claim 1 including:

selectively variable current means, independent of the source of said radio-frequency currents, for varying the inductance of the inductive portion of said coupler means by changing the effective permeability of said ferrite material.

3. The antenna defined in claim 1 wherein the capacitive and inductive portions of said coupler means are connected in series and said radio-frequency currents are applied to the junction between said inductive portion and said capacitive portion of said coupler means.

4. An antenna as defined in claim 3 including:

a tuning capacitor connected in series between the source of said radio-frequency currents and said junction.

5. An antenna as defined in claim 3 including:

means to selectively vary the capacitance of the capacitive portion of said coupler means.

6. An antenna, comprising:

a generally U-shaped conductive frame defining a notch;

a conductive sheet secured to the periphery of said frame and extending outwardly therefrom for radiating radio-frequency signals;

a ferrite-loaded inductive means;

capacitive means connected in series with said inductive means, and the series combination of said inductive means and said capacitive means being shunted across opposing sides of said U-shaped frame; and

a two-conductor transmission line having one of its conductors connected to the junction of said capacitive means and said frame, and the other of its conductors coupled to the junction between said inductive means and said capacitive means.

7. An antenna as defined in claim 6 wherein said U-shaped frame is mounted within an aperture in an airframe with the open end of said U-shaped frame directed outwardly therefrom.

8. An antenna as defined in claim 6 wherein said conductive sheet comprises the skin of an aircraft.

9. An antenna as defined in claim 6 wherein said ferrite-loaded inductive means comprises:

an elongated cylindrical metallic conductor; and

a hollow ferrite cylinder coaxially disposed with respect to said metallic conductor.

10. An antenna adapted to be mounted in a cutout in the external surface of an aircraft, comprising:

a generally U-shaped conductive frame defining a notch mounted in said cutout and having its open end flush with said exterior surface;

a ferrite-loaded inductive element located within said frame;

a first capacitor connected in series with said inductive element and the series combination of said first capacitor and said inductive element being shunted across opposing sides of said U-shaped frame;

a second capacitor having first and second terminals, said first terminal being connected to the junction between said inductive element and said first capacitor;

7 8 a two-conductor transmission line having one of its desired changes in operating frequency of said an conductors connected to said second terminal of tenna. said second capacitor and the other of its conduc- 14. An antenna as defined in claim 10 wherein said tors connected to the junction of said first capacitor inductive element comprises: and the side of said frame; and 5 an elongated conductor; and a dielectric window covering said cutout. a hollow ferrite cylinder 'coaxially disposed with re- 11. An antenna as defined in claim 10 wherein said spect to said elongated conductor means. series combination is located adjacent the open end of said U-shaped frame. References Cited 12. An antenna as defined in claim 10 including: 10 UNITED STATES PATENTS a g i gggfig at least a Porno of sad mducnve 2,903,000 10/1959 Robinson 343 746 a source of selectively variable direct current connected FOREIGN PATENTS to said coil for producing magnetic flux therein, and thereby modify the effective inductance of said 15 10/1958 Great Bntam' mductlve element ELI LIEBERMAN, Primary Examiner 13. An antenna as defined 1n clalm 10 mcludrng: means for selectively varying the capacitance of said US, Cl, X R

first and said second capacitors in accordance with 343 -746, 787, 789 

